Wide-bandwidth chirped fiber bragg gratings with low delay ripple amplitude

ABSTRACT

A device including a chirped Bragg grating, said grating having (a) a reflection bandwidth having a full-width at half maximum that is greater than 6 nm, and (b) a reflection delay ripple amplitude of less than ±50 ps.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of U.S. Ser. No. 09/809,393,filed Mar. 15, 2001, now pending, the disclosure of which is hereinincorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention relates generally to a device used forchromatically dispersing lightwave signals in fiber optics, and morespecifically to a wide-bandwidth chirped fiber Bragg grating that has alow delay ripple amplitude.

BACKGROUND OF THE INVENTION

[0003] Modern communication system providers are striving to increasethe capacity of their systems to satisfy the rapidly growing exchange ofinformation around the world. Increasing the data rate of a singlewavelength channel is one strategy to increase the throughput on opticalfibers. However, this approach is limited in that the data rate for asingle optical channel will eventually reach practical limitations. Animportant strategy to further increase the available bandwidth is to addmultiple wavelength channels. Multiple wavelength systems are referredto as being wavelength division multiplexed (WDM).

[0004] Optical communications systems are available with single-channeldata rates at 10 Gbit/s and faster. To accommodate the spectralbandwidth of these signals, the channels in a WDM system are commonlyspaced at 100 GHz, or ˜0.8 nm in the 1550 nm wavelength range. A devicewould have to be useful over bandwidths greater than ˜0.8 nm to be atruly multi-channel device in these WDM systems. Ideally a device wouldoperate over a full communications band of wavelengths, so systems couldbe designed for any WDM or modulation scheme without needing toaccommodate a specific dispersion correction module. Currentcommunications bands are defined by optical amplifier operating ranges;for instance, the “C” band covers ˜1530 nm to ˜1560 nm and the “L” bandcovers ˜1570 nm to ˜1610 nm.

[0005] In these optical communications systems, short pulses of opticalenergy are sent through optical fibers to transmit information. Theseoptical data pulse are comprised of a spectrum of wavelengths. Generallyspeaking, an unchirped pulse of duration τ has a spectral width of ˜1/τ,e.g., a ˜1 nanosecond (10⁻⁹ second) pulse has a ˜1 GHz (10⁹ Hz) spectralwidth. As a pulse travels along standard singlemode fiber in the ˜1550nm range, the shorter wavelength components travel faster than thelonger-wavelength components. This effect, called chromatic dispersion,broadens the pulse to the point that it eventually interferes withneighboring pulses in a pulse train and introduces errors in thedetected data stream. A number of solutions have been proposed for thisproblem, but only dispersion-compensating fiber (DCF) and chirped fibergratings have been considered seriously as potential candidates fordeployment.

[0006] Dispersion-compensating fiber has high levels of dispersion ofopposite sign to that of standard fiber. To compensate for thedispersion introduced by an 80-km span of standard fiber, one would haveto concatenate a ˜16-km length of DCF into the system. Thesecompensation modules are bulky, and due to the fiber design, suffer highoptical attenuation and increased optical nonlinear effects. However,DCF is used today since no serious alternative exists.

[0007] Fiber Bragg Gratings (FBGs) have emerged as a promising solutionfor dispersion compensation. An FBG is an optical fiber or other opticalwaveguide with periodic, aperiodic or pseudo-periodic variations of therefractive index along its length in the light guiding region of thewaveguide. Gratings are usually written in optical fiber via thephenomenon of photosensitivity. Photosensitivity is defined as theeffect whereby the refractive index of the glass is changed by actinicradiation-induced alterations of the glass structure. The term “actinicradiation” includes visible light, UV, IR radiation and other forms ofradiation that induce refractive index changes in the glass. Typicallyan interferogram of UV radiation is made and then a photosensitive fiberis placed into it. The period of the resulting FBG in the fiber is theperiod of the interferogram scaled by the waveguides refractive index.

[0008] To function as a dispersion compensator, the grating period of anFBG is chirped to reflect lagging wavelengths before faster wavelengths,which must travel further into the grating before they are reflected. Anoptical circulator is used to separate the input of the device from theoutput. A dispersion compensating grating (DCG) module recompresses adata pulse that had been corrupted by chromatic dispersion, and opticalsystem performance is enhanced. The longer the grating, the greater theDCG compression factor and the wider the bandwidth of the device.

[0009] As a practical matter, long length gratings for dispersioncompensation are not available, since extreme tolerances must bemaintained to manufacture quality long length gratings. Fabricationerrors in chirped gratings create ripples in the group delay curve andthus inaccuracies in the dispersion correction. The impact of theseripples on optical system performance is poorly understood, but somesystem designers have predicted that these ripples must be less than ˜40ps peak-to-peak for a DCG to be useful as dispersion compensators inmost systems. However, the magnitude of the ripple needed to make auseful FBG dispersion correction device has not been verified. A rippleamplitude of ˜40 ps peak-to-peak can be caused by a 20% variation in theFBG UV-induced index change, a ˜0.3% dimensional change in a fiber core,or a ˜4 pm error in grating pitch. Given that the silicon-oxygeninter-atomic spacing in glass is ˜160 pm, it has been widely believedthat holding these tolerances during grating inscription is notpossible, and that fiber fabrication tolerances are limiting the qualityof the gratings that they produce.

[0010] In 1995, a Swedish research group reported the fabrication of along-length FBGs by stitching smaller FBGs together. A small grating waswritten, the fiber was translated by a grating period through aUV-interferogram with a high-precision linear stage, and then the fiberwas irradiated again. This process was continued until a grating of thedesired length was made. With their system, this group reported itfabricated gratings of up to 50 cm in length. Since this announcement,other groups have extended this work and have reportedly fabricatedgratings up to 2.5-m-long. The range of motion of availablehigh-precision staging has limited the length these FBGs.

[0011] Several groups have adapted stitching methods to make chirpedlong length gratings, but stitching errors have caused these gratings tohave delay ripple amplitudes that are far too large for use asdispersion compensators in optical communications systems. To implementa stitching technique, one must have precise knowledge of a fiberlocation relative to the writing interferogram. The accuracy of locationmeasurements is limited by the motion stage encoder—usuallyinterferometer based, which is susceptible to several degradations, suchas interpolator inaccuracies, noise in edge detection electroniccircuitry, and random fluctuations in received interpolator-laser light.

[0012] Several feasibility studies have been completed where long-lengthFBGs, fabricated by stitching, have been used successfully at specificwavelengths as dispersion compensators in optical communication systems.Since the FBG delay ripple imposed very large distortion-derived systempenalties at most wavelengths, the wavelength of the transmitting laserin the communication system had to be adjusted in these studies toobtain reasonable system performance.

[0013] A common procedure for determining chromatic dispersion of adevice is the modulation-phase shift method, as described in Chapter 12of Fiber Optic Test and Measurement (ed. D. Derickson, Prentice HallPTR, NJ, 1998, ISBN #0-13-534330-5). The output of a narrowband, tunableoptical source is intensity modulated and applied to the device undertest. The transmitted (or reflected) signal is detected and the phase ofits modulation is measured relative to the electrical modulation source.The phase measurement is repeated at intervals across the wavelengthrange of interest. The curve of the relative group delay is constructedby accumulating these group delay changes across the measurementwavelength range.

[0014] The group delay ripple is determined by fitting with leastsquares minimization a line or a low-order polynomial to the relativegroup delay curve, and then this polynomial is subtracted from thecurve. The remainder of the subtraction is the delay ripple. Typicallythis ripple is considered as being “high frequency” ripple, i.e. ripplewith a periodicity of less than the channel bandwidth of acommunications system, and “low frequency” ripple, i.e. ripple with aperiodicity greater than that of a channel bandwidth. Hereinhigh-frequency ripple will be considered as ripple with a periodicity ofless than 80 pm (10 GHz @ ˜1550 nm), and low-frequency ripple will beripple with a periodicity of greater than 80 pm. The high-frequencyripple adds an intra-pulse distortion to a communication signal that isdifficult to correct, thus the high-frequency ripple is considered morecritical than the low-frequency ripple, which merely adds a slight errorto the dispersion correction.

[0015] Establishing a correlation between delay ripple amplitude andoptical system performance has been confusing because different groupsmeasure DCGs in different manners and, often, are not explicit abouttheir measurement procedures.

[0016] Although several studies have demonstrated that DCGs could beused as dispersion compensators for a single communication channel,useful wide bandwidth devices have not been demonstrated, mainly becausethe delay ripple amplitude of these devices was too large. Widelychirped DCGs with several WDM channels operating across their bandwidthhave been demonstrated, but because the wavelength of the transmittinglaser in the communication system needs adjustment in all of thesestudies to obtain reasonable performance, these DCGs proved to have onlyvery narrow sections of usable bandwidth across their entire bandwidth.If a wideband DCG with a large delay ripple is useful only over a verynarrow range as a dispersion compensator, then the utility of thiswidely chirped device is lost. Others have demonstrated DCGs inlaboratory systems where DCGs have been used to correct the dispersionin several channels of a communication system, but a DCG that coversonly part of a communication band has limited appeal, since the systemmust be specially engineered to accommodate such a device. Gratings witha bandwidth of at least a third of a communications band (˜10 nm) orhalf a band (˜15 nm) have some appeal, since fewer accommodations forthe device must be made to use it in a communications system thannarrower devices.

[0017] Although DCF is used widely today to solve chromatic dispersionproblems in high-speed optical communications systems, new DCF designsmust attempt to match their dispersion and dispersion slope to beopposite to that of a given transmission fiber. But these DCF designsdon't exactly match the dispersion characteristics of their intendedfiber and thus leave a residual dispersion that accumulates overmultiple spans of transmission fiber. Since DCF designs can becomplicated and difficult to manufacture, several transmission fibers donot yet have a matching DCF solution, and some fibers, due to theircomplex dispersion characteristics, seem unlikely that a perfectlymatching DCF design is possible across a wide bandwidth.

[0018] There thus exists a need for wide bandwidth (i.e., greater thanseveral WDM channel spacings and preferably a full communications band)fiber Bragg gratings that can compensate for the chromatic dispersionand dispersion-slope in lightwave communications systems across itsbandwidth. There also exists a need in the art for a wide-bandwidthchirped fiber Bragg grating that has a low delay ripple amplitude (i.e.,<±50 ps). These and other needs are met by the present invention, ashereinafter described.

SUMMARY OF THE INVENTION

[0019] In one aspect, the present invention relates to a device used forchromatically dispersing lightwave signals in fiber optics, and to amethod for using this device to achieve chromatic dispersion. The devicecomprises a wide-bandwidth (>0.8 nm) chirped fiber Bragg grating thathas a low delay ripple amplitude (<±50 ps). These devices are useful aschromatic dispersion correction devices in high frequency (e.g., 10Gbit/s) optical communications system. In contrast to previously knowndevices used for this purpose, these devices do not exhibit largefluctuations in system performance penalty across their bandwidth.

[0020] In another aspect, the present invention relates to a method andapparatus for fabricating long length FBGs. In accordance with thisaspect of the invention, the fiber being written on is translated with˜1 ppm (10⁻⁶) velocity control. This is preferably accomplished byplacing the fiber into a helical groove on a spool that is mounted to arotary spindle. The spindle is rotated at a constant velocity, which ispreferably regulated by a large flywheel. The groove on the spool actslike a thread on a screw so that, as the spindle turns, a linear stageis used to track the laser beam on the fiber, in a manner analogous tocutting threads on a lathe. Gratings that are tens of meters long can befabricated with this method.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a schematic diagram of a long length fiber Bragg gratingfabrication system;

[0022]FIG. 2 is a schematic diagram of the motor system used to rotatethe helical spool at a constant angular velocity;

[0023]FIGS. 3a and 3 b show delay ripple spectra measured from twodifferent gratings fabricated with the system operating at differentlevels of angular momentum;

[0024]FIG. 4 shows how the measured delay ripple of a grating can varyas a function of the modulation frequency used with the modulation-phaseshift measurement method;

[0025]FIG. 5 shows reflection and reflection delay spectra of a highquality chirped long length grating;

[0026]FIG. 6 shows reflection delay ripple spectra of a high qualitychirped long length grating;

[0027]FIG. 7 is a schematic diagram of an optical system;

[0028]FIG. 8 is a graph that shows the bit-error-rate versus opticalsignal-to-noise measured in an optical communication system utilizing alow quality fiber Bragg grating as a dispersion compensator;

[0029]FIG. 9 shows an example eye diagram measured in an opticalcommunication system utilizing a high quality fiber Bragg grating as adispersion compensator;

[0030]FIG. 10 is a graph that shows the bit-error-rate versus opticalsignal-to-noise measured in an optical communication system utilizing ahigh quality fiber Bragg grating as a dispersion compensator;

[0031]FIG. 11 is a graph that shows reflection delay ripple spectra of ahigh quality chirped long length grating of ˜30 nm bandwidth;

[0032]FIG. 12 is a graph that shows the dispersion slope of a highquality chirped long length grating of ˜30 nm bandwidth;

[0033]FIG. 13 is a graph that shows the delay ripple of a high qualitychirped long length grating of ˜30 nm bandwidth; and

[0034]FIG. 14 is a graph that shows the optical signal to noise rationeeded to maintain the bit-error-rate between 10⁻⁹ and 10⁻¹⁰ across theDCG bandwidth.

DETAILED DESCRIPTION OF THE INVENTION

[0035] As used herein, the term “reflection delay ripple amplitude”refers to the amplitude measured by subtracting a sixth-order polynomialfrom the reflection delay curve measured by the modulation-phase shiftmethod and using a frequency of 200 MHz. Herein high-frequency ripplewill be considered as ripple with a periodicity of less than 80 pm (10GHz @ ˜1550 nm), and low-frequency ripple will be ripple with aperiodicity of greater than 80 pm.

[0036] As used herein, the term “full width at half maximum” or “FWHM”,when used in reference to a reflection bandwidth, means the range ofwavelengths reflected by the device with an amplitude of at least >50%of the maximum reflection amplitude (3 dB points).

[0037] In accordance with the present invention, a new FBG fabricationtechnique is provided that overcomes long-length FBG fabricationlimitations. Chirped long-length FBGs can be made in accordance withthis technique that exceed 10 m in length. This technique involves aprocess that translates a fiber at a precise velocity past a stationaryinterferogram made with a laser beam that is intensity-modulated atfrequency ƒ. In this method, the fiber is treated as the recordingmedium of an analog signal, analogous to magnetic tape recording where amagnetic medium is translated at a controlled velocity past a magneticcircuit writing head. The radiation dose Φ delivered to a fiber alongits length χ as it is translated at velocity ν across the laser beam maybe expressed as${\Phi (x)} \propto {1 - {\frac{1}{2} \cdot {{\cos \quad\lbrack {2\quad {\pi \cdot \frac{f}{v} \cdot x}} \rbrack}.}}}$

[0038] Assuming that the refractive index perturbation induced in thefiber is proportional to the delivered dose of radiation, a grating ofperiod v/f will be written into the fiber. This result is significant,since the methodology allows for the manufacture of a chirped FBG of anylength by changing the frequency of the light amplitude modulation orthe velocity of the fiber. Complex FBGs can be written into the fibersimply by programming that function into the computer that controls thelaser beam modulation.

[0039] Past approaches have relied on measuring with extreme accuracythe position of a fiber relative to an interferogram to stitch gratingstogether. However, the accuracy of location measurements is limited bythe motion stage encoder—usually interferometer based—which issusceptible to several degradations, such as interpolator inaccuracies,noise in edge detection electronic circuitry, and random fluctuations inreceived interpolator-laser light. The advantage of the presenttechnique over previous methods is that it is a velocity-controlledapproach that does not require position information for feedback, andthus is not limited by the throw of available high-precision motionstaging.

[0040] To meet the accuracy requirement for fabricating quality DCGs,the fiber should be translated with ˜1 ppm (10⁻⁶) velocity control. Thisis preferably accomplished by placing the fiber into a helical groove 50on a spool 180 that is mounted to a rotary spindle, as shown in FIG. 1.The spindle is rotated at a constant velocity, preferably by using alarge flywheel to regulate the speed. The groove on the spool acts likea thread on a screw so that, as the spindle turns, a translation motionstage is used to track the laser beam on the fiber in a manner analogousto cutting threads on a lathe. Gratings that are tens of meters long canbe fabricated with this method. To meet the tolerances needed tofabricate DCGs, the spool diameters are preferably trued to <10millionths of an inch (250 nm) precision.

[0041] A detailed view of the machine used to rotate the spool 180 at aconstant angular velocity is shown in FIG. 2. An air bearing 240 is usedas the rotational point, which is mounted to an outside fixed housing280. A flywheel 230 is used to regulate the rotational speed. A mountingchuck 220 attaches the writing spool 180 to the flywheel 230 and the airbearing 240. The spindle system is driven by an induction motorconsisting of a stator 260 that is attached to the ouside housing 280and a rotor made of a high-permeable metal core 250 with a highresistive conductivity metal shell 270 press-fit on it. A rotationalencoder 290 is mounted to the housing 280 and the rotor core 250 toprovide velocity information to the control electronics.

[0042] It has been found that constant velocity can be maintained to adegree that exceeds available position measurements by increasingappropriately the momentum of the motion system to mechanically low-passfilter interpolator errors. It has also been found that the angularmomentum of a rotary fabrication system can be increased to reduce theeffect of outside disturbances on the fabrication system and producequality DCGs.

[0043] An air-bearing spindle, which exhibits relatively no torqueripple, supports the motor in the fabrication system. Without wishing tobe bound by theory, it is believed that turbulence from the air in thisspindle (and other vibrations in other support structures) causes errorsin the fiber motion and thus compromises the grating quality. Theangular momentum L of a symmetrical rigid body may be expressed as L=Iω,where I is the rotational inertia and ω is the angular velocity of therotating body. By increasing the system rotational speed and byincreasing its rotational inertia, chirped FBGs have been fabricatedwith decreased delay ripple amplitudes. Preferably, the rotationalinertia of the system is at least 1 g cm², more preferably, at least 10g cm², and most preferably, at least 100 g cm².

EXAMPLE 1

[0044] This example illustrates the effect of increasing the rotationalinertia, and hence the angular momentum, of the spindle used in the DCGfabrication system.

[0045] Two chirped FBGs were written, and their delay ripple amplitudewas analyzed. The fabrication system of the first FBG comprised aninduction motor with a smooth-walled rotor supported by an air bearingspindle. The spindle was controlled to rotate at a constant velocitywith phase-locked-loop (PLL) electronics that followed a moiré-effectrotary encoder. A spool with a helical groove to hold the optical fiberwas also mounted to the air-bearing spindle. A spectrum of the delayripple is shown in FIG. 3a. The delay ripple amplitude was hundreds ofpicoseconds. The rotaional inertia of the first system was ˜0.5 g cm.

[0046] The fabrication of the second gratings was the same as the first,but in this case the rotational inertia of the system was increased 40times by adding a 40-cm-diameter flywheel to the system. The inertia ofthe first system was ˜220 g cm². A spectrum of the delay ripple of thesecond FBG is shown in FIG. 3b. The delay ripple amplitude issubstantially reduced from that of the FBG made without the flywheel.

[0047] The gross deviation from the baseline with a periodicity of a fewnanometers, or very low-frequency ripple, is due to the poor quality ofthe encoder used in these experiments and is irrelevant for the point ofthis demonstration.

EXAMPLE 2

[0048] A measurement of a DCG delay ripple with the modulationphase-shift method can vary depending on the modulation frequency, asshown in FIG. 4. If the measurement is made at 1 GHz, a ripple with ½the amplitude of that measured with a 100 MHz signal may be determined.It has been found that measuring the devices with a modulation frequencyof <200 MHz gives consistent results. In some cases where theperiodicity of the ripple is very fine, a frequency of <50 MHz isneeded. For the measurements reported herein, a modulation frequency of200 MHz was used as a standard.

EXAMPLE 3

[0049] This example further illustrates the improvements possible indelay ripple amplitude through the use of increasing the angularmomentum of the spindle.

[0050] The fabrication system detailed in Example 1 was further improvedby installing a heavier flywheel attached to the system of inertia ˜650g cm². The gratings were also written at faster rotational speeds tofurther increase the angular momentum of the fabrication system. Theresulting FBGs exhibited delay ripple amplitudes of less than ±30 psover bandwidths greater than 1 nm. The reflection, delay in reflection,and delay ripple of such a grating is shown in FIGS. 5 & 6.

EXAMPLE 4

[0051] The performance of the dispersion compensating grating (DCG) wasassessed in a system. The tested configuration, which is shown in FIG.7, consisted of a transmission line of 40 km conventional single modefiber (SMF) with a nominal dispersion of +17 ps/nm/km. Light from acommercially available tunable laser with tuning capability in steps of1 pm was modulated with 2²³−1 pseudo-random binary sequence (PRBS) 10Gb/s nonreturn-to-zero (NRZ) data via a chirp free external LiNbO₃modulator. The 10 Gb/s optical data output from the modulator wasamplified by an Er⁺-doped fiber amplifier (EDFA) and launched into the40 km SMF transmission line. The launched optical power into the SMF waskept low (˜2 dBm) in order to avoid any possible nonlinear effects.After transmission the signal was again amplified by a second EDFAcompensating the attenuation in SMF.

[0052] DCGs with a dispersion of −680 ps/nm were placed after the secondEDFA to compensate the total accumulated dispersion of the transmissionline. The dispersion compensated signal was directed to a 10 Gb/sreceiver where the optical data was converted into an electrical 10 Gb/sdata stream and a clock signal was recovered. A tunable optical bandpassfilter of 0.9-nm bandwidth, which followed the source wavelength, wasused before the receiver to suppress the ASE noise outside the signalbandwidth, hence improving the receiver performance.

[0053] As an example of a poorly performing DCG, FIG. 8 shows thebit-error-rate (BER) versus optical signal-to-noise ratio (OSNR) where aDCG with ˜±50 ps delay ripple amplitude is used as a dispersioncompensator in an optical fiber communication system. Notice that as thewavelength of the signal is changed slightly, the BER performance at agiven OSNR varies by orders of magnitude, rendering the device useless.

[0054] In contrast, the DCG illustrated in Example 3 was placed in thesystem. FIG. 9 shows eye patterns of data signal, recorded with asampling oscilloscope, at various locations along the transmission line.The dispersion induced eye closure at the end of 40 km of SMF and itscomplete restoration at the receiver clearly shows the effectiveness ofthe dispersion compensation by the DCG.

[0055] The uniformity of the DCG performance in the system was examinedby measuring bit-error-rate (BER) as a function of optical signal tonoise ratio (OSNR) across the DCG bandwidth. The measurement involvedadding noise to the transmitted signal after dispersion compensation,effectively varying the OSNR, and measuring the resulting BER using a 10Gb/s bit-error-rate testset (BERT). The noise source used in the systemwas ASE noise generated from a combination of two EDFAs. An opticalbandpass filter was used to limit the ASE bandwidth of the first EDFA tomatch the bandwidth of the DCG. The ASE noise after filtering thusbehaves as a pump signal to the second EDFA generating a high power ASEnoise within the bandwidth of interest. The ASE noise was summed withthe transmitted signal through an attenuator, and the OSNR, measuredwith an optical spectrum analyzer, was varied by varying the attenuator.FIG. 10 shows the results of these measurements. It can be clearly seenthat as the source wavelength was changed across the DCG bandwidth theOSNR needed to obtain a given BER varies by <1 dB. No wavelengths werefound throughout the DCG bandwidth that did not operate in a similarmanner. Eye diagrams and BER tests are commonly used tools to assess acommunications systems performance, as described in Chapter 8 of FiberOptic Test and Measurement (ed. D. Derickson, Prentice Hall PTR, NJ,1998, ISBN #0-3-534330-5).

EXAMPLE 5

[0056] A wider bandwidth grating was fabricated with the FBG fabricationsystem described in Example 4. Spectra from an example wideband FBG of˜2 m length is shown in FIG. 11, where the reflectivity and delay of thedevice is shown. The gross insertion loss of this device is typically ˜5to 5.5 dB (including circulator loss), and the insertion loss variationsis ˜±1 dB. The insertion loss of the grating measured in transmissionis >3 dB. The bandwidth of the shown device is >30 nm. After subtractingthe linear dispersion of −629 ps/nm from the delay curve, the delayslope of −1.1 ps/nm² remains, as shown in FIG. 12. After fitting aquadratic polynomial to the dispersion slope curve shown in FIG. 12, thedelay ripple remains, as shown in FIG. 13. These measurements were madewith the modulation-phase shift method at a frequency of 200 MHz at a ˜5pm resolution. A swept tunable laser system, where the scan rate of thelaser has been synchronized with the network analyzer, performed thesemeasurements. Due to the nature of this measurement configuration, thedelay ripple is effectively averaged over a bandwidth of ˜5 pm. Notethat the high frequency ripple of this device is less than ±25 ps acrosslarge bandwidth regions of the device, and is less than ±40 ps acrossthe FWHM of the device.

[0057] The performance of this DCG was assessed in a system testbed,described in Example 4. The uniformity of the DCG performance in thesystem is examined by measuring the optical signal to noise ratio (OSNR)required to maintain a fixed bit-error-rate (BER) across the DCGbandwidth. Noise was added to the transmitted signal after dispersioncompensation to vary the OSNR, and the BER was determined. As the systemwavelength was varied across the DCG bandwidth in 25 pm steps, the noiseamplitude was adjusted to keep the BER in the range from 10⁻⁹ to 10⁻¹⁰.Noise, generated by combining the ASE from two EDFAs, was added to thetransmitted signal through an attenuator, and the OSNR, measured with anoptical spectrum analyzer, was varied with the attenuator. FIG. 14 showsthe OSNR needed to maintain the BER across the DCG bandwidth.

[0058] Those skilled in the art will appreciate that the presentinvention may be used in the manufacture of a variety of opticalcomponents. While the present invention has been described with areference to exemplary preferred embodiments, the invention may beembodied in other specific forms without departing from the spirit ofthe invention. Accordingly, it should be understood that the embodimentsdescribed and illustrated herein are only exemplary and should not beconsidered as limiting the scope of the present invention. Othervariations and modifications may be made in accordance with the spiritand scope of the present invention.

What is claimed is:
 1. A device, comprising: a chirped Bragg grating,said grating having (a) a reflection bandwidth having a full-width athalf maximum that is greater than 6 nm, and (b) a reflection delayripple amplitude of less than ±50 ps.
 2. The device of claim 1, whereinsaid device has a dispersion, measured in reflection, whose magnitude isgreater than 100 ps/nm.
 3. The device according to claim 1, wherein saiddevice has a dispersion, measured in reflection, whose magnitude isgreater than 400 ps/nm.
 4. The device of claim 1, wherein said devicehas a peak insertion loss, measured in transmission, of greater than 0.1dB.
 5. The device according to claim 1, wherein said device has a peakinsertion loss, measured in transmission, of greater than 1 dB.
 6. Thedevice according to claim 1, wherein the device has a reflectionbandwidth greater than 10 nm.
 7. The device according to claim 1,wherein the device has a reflection bandwidth greater than 15 nm.
 8. Thedevice according to claim 1, wherein the device has a delay rippleamplitude A such that |A|<30 ps.
 9. The device according to claim 1,wherein the bandwidth of the device is greater than 25 nm.
 10. Thedevice according to claim 1, wherein the device has a high-frequencydelay ripple amplitude A such that |A|<50 ps.
 11. The device accordingto claim 1, wherein the device has a high-frequency delay rippleamplitude A such that |A|<30 ps.
 15. An optical fiber for propagatinglight comprising a core; a cladding; and a chirped Bragg gratingcomprising refractive index perturbations formed within the waveguide,said grating having a bandwidth in reflection having a full-width athalf maximum greater than 6 nm; a reflection delay ripple amplitude,determined by subtracting a sixth-order polynomial from the reflectiondelay curve measured by the modulation-phase shift method with afrequency of 200 MHz, less than ±50 ps; a dispersion, measured inreflection, of magnitude greater than 100 ps/nm; and a peak insertionloss, measured in transmission, greater than 1 dB.
 16. An opticaldevice, comprising: a waveguide equipped with a chirped Bragg grating,said grating comprising refractive index perturbations formed within thewaveguide and having a bandwidth in reflection having a full-width athalf maximum greater than 6 nm; and a reflection delay ripple amplitudewhich, when determined by subtracting a sixth-order polynomial from thereflection delay curve measured by the modulation-phase shift methodusing a frequency of 200 MHz, is less than k; wherein |k|<50 ps.
 17. Anoptical communications system that uses the device of claim
 1. 19. Anoptical communications system that uses the optical fiber of claim 15.20. An optical communications system that uses the device of claim 16.